Decomposition of Xylose in Sub-and Supercritical Water

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Industrial & Engineering Chemistry Research

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Decomposition of xylose in sub- and supercritical

2

water

3

Nattacha Paksung1, Yukihiko Matsumura2* 1

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Department of Mechanical Sciences and Engineering, Hiroshima University Division of Energy and Environmental Engineering, Hiroshima University

*

To whom correspondence should be addressed. Fax: +81-82-422-7193. E-mail:

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[email protected].

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Abstract: The purpose of this study was to elucidate the decomposition characteristics of

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xylose, a model compound for hemicellulose, in subcritical and supercritical water. The

10

experiment was carried out at temperatures of 300–450 °C, a pressure of 25 MPa, and a

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residence time of less than 7 s; xylose decomposed rapidly, but it was still detected at a

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temperature of 300 °C. Furfural and retro-aldol condensation products were found to be the

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major liquid intermediates. A reaction network was proposed and the kinetics parameters of

14

all reactions were calculated on the basis of data fitting, assuming that all reactions are first-

15

order. Finally, the temperature effect was used to classify the reactions as radical reactions

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(showing Arrhenius behavior in the supercritical region) or as ionic reactions (not showing

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Arrhenius behavior in the supercritical region).

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Keywords: xylose, kinetic model, supercritical water gasification (SCWG), biomass

ACS Paragon Plus Environment


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1. Introduction

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Recently, the depletion of fossil fuels has become a global environmental issue,

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because the global energy consumption is increasing while energy resources are not sufficient

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to satisfy the demand. Therefore, renewable energy has attracted attention to overcome this

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issue and to achieve sustainable development. Biomass-derived energy is one candidate

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because it is carbon neutral and, therefore, environmentally friendly. Because most of the

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biomass resources contain much moisture, supercritical water gasification (SCWG) is a

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promising method to convert biomass into gaseous products; water is employed as a reactant

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in the process, so there is no need to dry the biomass beforehand. Water is supercritical when

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both the temperature and the pressure are above their critical values (374 °C and 22.1 MPa,

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respectively). At this state, water has a high potential as a solvent for organic components

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and gases, because all fluids stay in a single phase 1–3. In addition, the density of supercritical

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water ranges between the densities of gaseous and liquid water, and its viscosity is much

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lower than that of liquid water 4. As a result, biomass can be homogeneously dissolved in

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supercritical water and high conversion and high hydrogen selectivity are obtained 5. Thus, it

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is a good reaction medium for biomass gasification 6–9. However, biomass consists of various

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compounds, which complicates the optimization of the process. Therefore, determination of

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reaction mechanism using model compounds for biomass is important.

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Lignocellulosic biomass is mostly composed of cellulose, hemicellulose, and lignin.

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Cellulose is the largest component of most of the biomass, but there are other components, so

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that their kinetics and reactions need to be studied. For example, the presence of lignin

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results in a decrease in the production of hydrogen, and is therefore undesirable

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acids are minor but common components of biomass, and they are not always easy to gasify

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11

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carbon sugar xylose, is likely to be converted into desirable products in a way similar to

10

. Amino

. On the other hand, hemicellulose, which is a heteropolymer mainly consisting of the five-


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cellulose, which contains six-carbon sugars. It was previously reported that, in subcritical

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water, furfural is the major decomposition product of xylose (used as a model compound for

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hemicellulose)

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condensation of D-xylose is dominant and dehydration to 2-furfural hardly takes place

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Aida et al. investigated the reaction kinetics of D-xylose in subcritical and supercritical water

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and found that D-xylulose is the primary product and intermediate for retro-aldol products

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and furfural 15. At a constant temperature, the kinetic rate was found to depend on the density

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of water. Goodwin and Rorrer proposed two models of reactions for the high-temperature

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supercritical water gasification of xylose: the xylose gasification kinetics model, which was

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used to predict the gas yield and composition, and the xylose decomposition kinetics model,

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which was used to predict liquid intermediates 16.

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12–13

, while in near-supercritical and supercritical water, the retro-aldol 14

.

However, detailed reaction mechanisms and corresponding reaction rates have not

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been reported.

This information is quite important when we consider the gasification

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characteristics of biomass, in which interactions between biomass components are expected.

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Thus, the purpose of this study is to thoroughly determine the intermediates involved in the

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decomposition of xylose under hydrothermal conditions in order to elucidate the

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decomposition mechanism as well as rates of the corresponding reactions. Furthermore, the

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classification of reactions as radical reactions or as ionic reactions in the SCWG of xylose

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was attempted by investigating the temperature dependence of the reaction rates.

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2. Experimental

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The experimental apparatus employed in this study is the same as that employed in

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previous studies by our research group 17. Briefly, it is composed of high-pressure pumps, a

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preheater, a reactor made of SS316 steel with inner and outer diameters of 1 mm and 1.59



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mm (1/16 inch), respectively, heat exchangers, a back-pressure regulator, and a gas- and

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liquid-sampling system. Both the preheater and heater were heated in a molten-salt bath.

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Deionized water was first fed into the preheater. A D-(+)-xylose (obtained from Nacalai

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Tesque, purity >98%) solution with a concentration of 7.5 wt% was prepared; it was fed to

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the reactor via a line different from that used for preheated water. The D-(+)-xylose solution

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and water were mixed in a volume ratio of 1:4 just before entering the reactor zone to avoid

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decomposition of xylose before reaching the reactor. Thus, the xylose solution was diluted to

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1.5 wt% when treated with preheated water. When the product emerged from the reactor, it

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was cooled down by the addition of room temperature water, after which it was further

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cooled down by heat exchange with tap water flowing in a cooling jacket. The products

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passed through an inline solid filter. At the final exit, the liquid effluent and gaseous

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products were collected at the liquid- and gas-sampling port. The pressure of the system was

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controlled by the back-pressure regulator.

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The experiment was carried out using the conditions shown in Table 1. All of the

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experimental runs were conducted at a pressure of 25 MPa. The temperature was varied

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between 300 and 450 °C. The reactor length was used to calculate the residence time. The

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gaseous product was analyzed by GC-TCD (gas chromatograph (GC) with a thermal

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conductivity detector) and GC-FID (GC with a flame-ionization detector) using He as the

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carrier gas; H2 was detected by GC-TCD using N2 as the carrier gas. The liquid effluent was

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analyzed by a total organic carbon (TOC) analyzer to quantify the amount of carbon

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compounds in the liquid product (non-purgeable organic carbon: NPOC) and dissolved

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gaseous products in the liquid (inorganic carbon: IC).

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chromatography (HPLC) was used to identify compounds in the liquid effluent. Remaining

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xylose, xylulose, glyceraldehyde, and glycolaldehyde were quantitatively detected with an

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SCR102H column (Shimadzu) using 0.005 M HClO4 aqueous solution as the mobile phase.


High-performance liquid

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For this column, the peak of dihydroxyacetone overlaps those of formic acid and lactic acid,

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which are possibly also present in the liquid product. Therefore, dihydroxyacetone was

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analyzed with an RSpak DE-413L column using 0.01 M H3PO4 as the mobile phase instead.

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Furfural was detected with an RSpak DE-413L column (Shodex) using 0.005 M HClO4

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aqueous solution and acetonitrile in a 1:1 volume ratio. The product yield of compound X was evaluated based on the carbon amount using

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the equation

YC ( X ) =

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nC ( X ) nC 0

(1)

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where nC ( X ) and nC 0 denote the amount of carbon in product X and that in the xylose

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feedstock, respectively.

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3. Results and discussion

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3.1.

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Product yield of gaseous and liquid products

Fig. 1 shows the obtained carbon yield of gaseous and liquid products. Gaseous

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products were rarely found under subcritical conditions (300 and 350 °C).

When the

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temperature was increased above its critical value (374 °C), the carbon yield of gaseous

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products slightly increased, but it was still much lower than that of liquid products. The

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carbon balance of each experiment exceeded 0.8. Unlike in the decomposition of glucose 18,

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the formation of char was hardly observed, showing that only a trace amount (if any) of solid

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product in the form of very small particles was formed. Hence, the formation of char was

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neglected in this study. This is in good agreement with the results of Chuntanapum and

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Matsumura

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low. When the temperature was above the critical point, the liquid product had a pale yellow

19

, who showed that the rate of polymerization of furfural to form char is very



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but almost transparent appearance, whereas the liquid product under subcritical conditions

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was clearly yellowish, which is possibly the color of furfural.

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3.2.

Decomposition products of xylose

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The decomposition behavior of xylose can be predicted on the basis of that of glucose,

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as reported in previous work 18. Because xylose has a molecular structure similar to that of

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glucose (xylose differs from glucose by the absence of a CH2O unit in the former), all

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glucose decomposition products reduced by a CH2O unit are possible intermediates involved

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in the decomposition of xylose. Considering the isomerization of glucose to fructose, the

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same reaction is similarly applied to the isomerization of xylose to xylulose. Analogous to

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the dehydration of glucose to 5-hydroxymethylfurfural (5-HMF), the conversion of xylose

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into furfural is a possibility. In addition, xylose is presumed to decompose mainly into retro-

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aldol products. Other liquid products, such as organic acids, are presumed to be formed from

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xylose, furfural, and retro-aldol products, after which they are finally gasified. Formaldehyde

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is already a small molecule (HCHO); it will not decompose into other liquid intermediates

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but into hydrogen and carbon monoxide.

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decomposition of xylose in supercritical water proposed in this study is presented in Fig. 2.

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All of these compounds were observed, as discussed below.

Hence, the reaction kinetic model for the

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3.3.

Liquid-phase decomposition products

According to previous reports

15–16

, the major refractory liquid intermediates are

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furfural (from the dehydration of xylose), retro-aldol condensation products, and organic

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acids. Under ionic reaction conditions below the critical point of water, xylose dehydration is


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favored. On the other hand, retro-aldol condensation is favored above the critical point of

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water, when free-radical reactions dominate.

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All the experimental results are shown in Table 2. The residence time showing in

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here is actual residence time calculated based on actual flow rate at each condition. The

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results obtained in this study are in good agreement with previous knowledge: the amount of

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furfural was relatively higher when the experiment was carried out at 300 and 350 °C than

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when the temperature was above the critical point of water. The yield of retro-aldol products

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was found to display a trend opposite to that of furfural, although not as obvious.

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Focusing on the retro-aldol condensation, the reaction forms intermediate products by

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breaking a C–C bond in xylose as well as xylulose. The cleavage occurs at the position

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between the carbon atom next to the carbonyl group, which is the alpha (α) carbon, and the

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atom next to the alpha carbon, which is the beta (β) carbon.

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condensation of xylose gives glycolaldehyde and glyceraldehyde; the latter may undergo the

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same reaction, producing glycolaldehyde and formaldehyde. In the case of xylulose, the C–C

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bond cleavage converts xylulose into glycolaldehyde and dihydroxyacetone. Such a

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mechanism is illustrated in Fig. 3.

Thus, the retro-aldol

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The experimental results are in good agreement with the assumption that retro-aldol

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condensation occurs during the decomposition of xylose in the SCWG. Among the retro-

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aldol condensation products, glycolaldehyde was found in the largest amounts.

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Glyceraldehyde was found in a very small amount under all conditions, while a relatively

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large amount of dihydroxyacetone was found in the subcritical region. Formaldehyde was

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abundant in the supercritical region.

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3.4.

Kinetic model



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Assuming that all reactions in the network are first-order, the change in yield for each

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compound and the reaction kinetics can be evaluated by calculation using the following

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differential equations:

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(2)

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(3)

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(4)

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(5)

172

173 174

(6)

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(7)

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(8)

177


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(9)

180

(10)

181

where YC, t, and k denote the carbon yield of each compound, reaction time, and reaction rate

182

constant, respectively.

183

Note that the yield of each product is based on the number of carbon atoms in each

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molecule. Therefore, the yield can be used to determine the kinetics parameters without

185

defining the molecular formula of TOC and gaseous products. In the case of Eqs. 6−8, the

186

coefficients were multiplied by the amount of product obtained from the retro-aldol

187

condensation in terms of the carbon balance. For instance, of the five carbon atoms of xylose,

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three end up in glyceraldehyde and two end up in glycolaldehyde. A similar method was

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applied to the retro-aldol condensation reactions of xylulose and glyceraldehyde.

190

Kinetic rate constants were determined by numerically integrating Eqs. 2−10 with the

191

assumed kinetic rate constants. The values that gave the best fits between the calculated and

192

experimental data were employed; the least square of error (LSE) was the criterion.

(11)

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[exp]

=

the experimental yield [-],

195

[cal]x

=

the calculated yield determined by the set of

196

kinetic parameters x.

194

Where



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197

The calculated yields and the experimental yields are shown in Fig. 4. The calculated

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data fit the experimental data quite well. In the temperature range studied here, xylose

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decomposes very fast (after a short residence time). Furfural and retro-aldol products were

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found to be the most abundant liquid products in the decomposition of xylose in the

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subcritical region; retro-aldol products prevailed over furfural in the supercritical region. The

202

yield of gaseous products increased with increasing residence time.

203

204 205

3.5.

Effect of temperature on reaction type

In previous work of Promdej and Matsumura

20–21

, the temperature effect on the

206

decomposition of glucose was studied, and the reactions were classified as ionic reactions or

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radical reactions on the basis of the temperature dependence of the reaction rate constants.

208

Under supercritical conditions, the influence of ionic reactions is lower because of the

209

reduced stability of ions in supercritical water; thus, ionic reactions are suppressed 22. On the

210

other hand, free-radical reactions play dominant roles in the decomposition of biomass under

211

supercritical conditions 23–24.

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The kinetic rate constants of all reactions obtained from this study are summarized in

213

Table 3. Arrhenius plots of all reactions are shown in Fig. 5. In this figure, the natural

214

logarithm of each rate constant is plotted as a function of the reciprocal absolute temperature.

215

Kinetic parameters with values of zero were rejected from the graph because of the limitation

216

imposed by the logarithm. The following reactions follow Arrhenius behavior because the

217

Arrhenius plots are linear: the retro-aldol condensation of xylose to glyceraldehyde and

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glycolaldehyde (kxgl), the retro-aldol condensation of glyceraldehyde to glycolaldehyde and

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formaldehyde (kglgc), the retro-aldol condensation of xylulose to glycolaldehyde and

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dihydroxyacetone (kxygl), the decomposition of xylose, xylulose, and glycolaldehyde to TOC


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(kxt, kxyt, kgct), and the gasification (ktg). Overall, the decomposition of xylose (kx) was found

222

to likely be dominated by radical reactions, thus displaying Arrhenius behavior. Activation

223

energies and pre-exponential factors of those reactions are shown in Table 4. In comparison

224

with previous work employing Arrhenius plot, the total xylose decomposition of present

225

study shows a consistent trend with those of the previous studies as shown in Fig. 6.

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However, some reactions do not obey Arrhenius behavior: the isomerization of xylose

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and xylulose (kxxy, kxyx), the isomerization of glyceraldehyde and dihydroxyacetone (kgld, kdgl),

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the dehydration of xylose and xylulose to furfural (kxf, kxyf), and the decomposition of

229

glyceraldehyde (kglt). The explanation for non-Arrhenius behavior in these cases is that in the

230

subcritical region, dissociated H+ and OH- promote acid- or base-catalyzed reactions in water

231

25

232

which they drastically decrease when the concentrations of ionic products decrease in the

233

supercritical region 23.

. Reaction rates for these reactions increase until the critical temperature is reached, after

234

For the decomposition of furfural to TOC (kft), the kinetic parameter did not show

235

clear trend as a function of temperature. However, it is assumed that this reaction is more

236

likely to be a radical reaction, because the kinetic parameter was found to be zero in the

237

subcritical region and higher in the supercritical region.

238

The other reactions constants, including those for the decomposition of formaldehyde

239

and dihydroxyacetone (kfog, kdt), were found to be zero at all temperatures. Therefore, it can

240

be concluded that these compounds are stable and do not decompose further into other

241

products.

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Each reaction could be classified as a free-radical reaction (which displays Arrhenius

243

behavior) or as a radical reaction (which does not display Arrhenius behavior). Table 5

244

summarizes the reaction classification.



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246 247

4. Sensitivity analysis Sensitivity analysis of individual kinetic rate constant that affect to product yield was

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conducted referring to a method of the previous study

249

coefficient, S, is defined as

S=

250

26

.

∂lnYC (X) ∆YC (X)/YC (X) = ∂lnk ∆k/k

The normalized sensitivity

(12)

251

The sensitivity of the yield of each product on 5% change on individual rate constant.

252

The calculation was done at two temperatures, near supercritical point at 350 oC and in

253

supercritical water at 450 oC and at the shortest and longest residence time of each condition.

254

All sensitivity coefficients are shown in Table 6. The sensitivity analysis at temperature of

255

350 oC indicates that yield of xylose is sensitive to retro-aldol reaction. Xylulose and furfural

256

are sensitive to formation reaction than decomposition reaction. Except for dihydroxyacetone,

257

glyceraldehyde, glycolaldehyde, and formaldehyde, which are retro-aldol products are

258

sensitive to retro-aldol reaction, while dihydroxyacetone is sensitive to isomerization of

259

glyceraldehyde. At last, gas product is sensitive to gasification of TOC, which might include

260

final intermediates that produce gas.

261

262

5. Conclusion

263

Xylose was hydrothermally decomposed using a flow reactor at temperatures of 350–

264

450 °C and a pressure of 25 MPa. Xylose decomposed rapidly, after a very short residence

265

time. Xylose was only detected in a sample at a temperature of 300 °C, which indicated that

266

the decomposition rate of xylose is comparatively low under subcritical conditions. When

267

the temperature was below the critical point of water, furfural was found to be the most


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abundant liquid product, followed by retro-aldol products. In contrast, the yield of retro-aldol

269

products, especially glycolaldehyde, was high in the supercritical region. The reaction rate

270

parameters of all reactions in the proposed reaction network were determined on the basis of

271

data fitting, assuming that all reactions are first-order. The effect of temperature was used

272

successfully to classify the reactions as radical reactions or ionic reactions. In addition,

273

sensitivity analysis was conducted.

274

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(20) Promdej, C.; Chuntanapum, A.; Matsumura, Y. Effect of Temperature on Tarry

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Material Production of Glucose in Supercritical Water Gasification. J. Jpn. Inst.

328

Energy 2010, 89, 1179.

329

(21) Promdej, C.; Matsumura, Y. Temperature Effect on Hydrothermal Decomposition of

330

Glucose in Sub- and Supercritical Water. Ind. Eng. Chem. Res. 2011, 50, 8492.

331 332

(22) Kruse, A.; Dinjus, E. Hot Compressed Water as Reaction Medium and Reactant. J. Supercrit. Fluids 2007, 39, 362.

333

(23) Antal, M. J.; Mok, W. S. L.; Roy, J. C.; -Raissi, a. T.; Anderson, D. G. M. Pyrolytic

334

Sources of Hydrocarbons from Biomass. J. Anal. Appl. Pyrolysis 1985, 8, 291.

335

(24) Kruse, A.; Gawlik, A. Biomass Conversion in Water at 330−410 °C and 30−50 MPa.

336

Identification of Key Compounds for Indicating Different Chemical Reaction

337

Pathways. Ind. Eng. Chem. Res. 2003, 42, 267.

338

(25) Akizuki, M.; Fujii, T.; Hayashi, R.; Oshima, Y. Effects of Water on Reactions for

339

Waste Treatment, Organic Synthesis, and Bio-Refinery in Sub- and Supercritical

340

Water. J. Biosci. Bioeng. 2014, 117, 10.



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341 342

(26) Guan, Q.; Wei, C.; Savage, P. E. Kinetic Model for Supercritical Water Gasification of Algae. Phys. Chem. Chem. Phys. 2012, 14, 3140.

343


Page 16 of 38

Page 17 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


344

Table Captions

345

Table 1. Experimental conditions

346

Table 2. Carbon yield of products from decomposition of xylose

347

Table 3. Calculated kinetic rate constant of each reaction in the network

348

Table 4. Activated energy and pre-exponential factor of Arrhenius reactions

349

Table 5. Classification of reactions in SCWG of xylose

350

Table 6. Sensitivity coefficients of xylose decomposition at temperature of 350 oC and 450

351

o

352

Figure Captions

353

Figure 1. Carbon balance: carbon content in liquid products and gaseous products, for the

354

experiments conducted at the temperature of (a) 300, (b) 350, (c) 400 and (d) 450°C

355

Figure 2. Proposed reaction network of xylose decomposition in sub- and supercritical water

356

Figure 3. Retro-aldol condensation of xylose and xylulose

357

Figure 4. Product yield at temperature of (a) 300 oC, (b) 350 oC, (c) 400 oC, (d) 450 oC and

358

pressure of 25 MPa as a function of residence time

359

Figure 5. Arrhenius plot of (a) kxgl (b) kxygl (c) kglgc (d) kxt (e) kgct (f) ktg (g) kx (h) kxxy (i) kxyx

360

(j) kgld (k) kdgl (l) kxf (m) kxyf and (n) kglt

361

Figure 6. Arrhenius plot of rate constant of the total xylose decomposition for literature

362

comparison

C

363



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

364

365

Tables and Figures Table 1. Experimental conditions

366

Feedstock

d-xylose

Temperature

300, 350, 400, 450 C

Pressure

25 MPa

Concentration of feedstock

7.5 wt%

Feedstock : water ratio by volume

1:4

Residence time

0.5-7 s

o

367


Page 19 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 2. Carbon yield of products from decomposition of xylose

368 temp [℃]

water density [g/ml]

residence time [s]

300

7.430E-01

300

Product yield [-] xylose

xylulose

furfural

glyceraldehyde

glycolaldehyde

dihydroxyacetone

formaldehyde

TOC

gas

C balance

1.292

2.974E-01

0.000E+00

1.590E-01

6.329E-03

9.255E-02

4.113E-02

0.000E+00

3.019E-01

3.150E-03

0.901

7.430E-01

2.533

1.740E-01

2.051E-01

1.673E-01

4.661E-03

1.211E-01

3.893E-02

0.000E+00

2.258E-01

2.874E-03

0.940

300

7.430E-01

4.332

2.723E-01

2.619E-01

1.072E-01

8.170E-03

1.402E-01

3.673E-02

0.000E+00

2.104E-02

2.572E-02

0.873

300

7.430E-01

6.390

1.502E-01

1.889E-01

1.127E-01

3.422E-03

1.146E-01

3.552E-02

0.000E+00

2.021E-01

1.555E-02

0.823

350

6.255E-01

1.231

2.770E-02

1.136E-01

1.786E-01

3.137E-03

2.658E-01

5.534E-02

0.000E+00

3.246E-01

2.840E-02

0.997

350

6.255E-01

1.886

5.793E-03

4.140E-02

1.434E-01

9.156E-04

2.792E-01

3.457E-02

4.683E-02

3.453E-01

2.296E-02

0.920

350

6.255E-01

1.692

1.157E-02

7.223E-02

1.088E-01

3.435E-03

2.910E-01

5.211E-02

2.651E-02

3.739E-01

2.846E-02

0.968

350

6.255E-01

3.818

2.647E-03

2.583E-02

1.221E-01

5.073E-04

1.810E-01

2.381E-02

4.129E-02

4.572E-01

2.152E-02

0.876

350

6.255E-01

5.714

4.152E-02

2.573E-02

1.397E-01

1.252E-02

1.998E-01

2.359E-02

1.202E-01

3.708E-01

3.870E-02

0.973

400

1.665E-01

0.850

2.092E-03

1.582E-02

2.365E-02

6.343E-04

3.804E-01

2.250E-02

1.414E-01

3.427E-01

5.039E-02

0.980

400

1.665E-01

1.051

1.944E-03

1.547E-02

1.656E-02

7.669E-04

3.809E-01

2.390E-02

1.358E-01

3.003E-01

3.584E-02

0.911

400

1.665E-01

1.285

1.661E-03

1.207E-02

1.528E-02

5.187E-04

3.022E-01

0.000E+00

1.311E-01

5.142E-01

3.158E-02

1.009

400

1.665E-01

3.325

2.312E-04

2.443E-03

1.680E-02

5.638E-02

1.722E-01

0.000E+00

1.046E-01

4.581E-01

1.681E-01

0.979

400

1.665E-01

5.446

1.214E-03

1.480E-02

1.844E-02

4.133E-04

2.330E-01

0.000E+00

1.088E-01

5.435E-01

3.243E-02

0.953

450

1.090E-01

0.716

1.395E-03

7.428E-03

2.182E-02

2.492E-04

2.477E-01

0.000E+00

1.149E-01

4.818E-01

7.517E-02

0.950

450

1.090E-01

0.963

3.692E-03

1.641E-02

2.329E-02

2.348E-04

3.842E-01

0.000E+00

1.475E-01

3.348E-01

6.909E-02

0.979

450

1.090E-01

1.194

4.488E-03

1.568E-02

2.716E-02

1.172E-04

3.421E-01

0.000E+00

1.309E-01

3.659E-01

5.403E-02

0.940

450

1.090E-01

3.091

1.679E-03

3.903E-03

2.284E-02

1.457E-05

1.158E-01

0.000E+00

9.008E-02

5.287E-01

1.145E-01

0.877

450

1.090E-01

5.097

2.627E-05

1.449E-03

2.750E-02

5.583E-05

9.962E-02

0.000E+00

7.517E-02

6.458E-01

1.534E-01

1.003



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

369

Page 20 of 38

Table 3. Calculated kinetic rate constant of each reaction in the network k [s-1]

Kinetic Reaction parameter

300 oC

350 oC

400 oC

450 oC

kxxy

Isomerization

1.60E-01

3.72E-01

2.39E-01

2.33E-01

kxgl

Retro-aldol

1.93E-01

1.49E+00

6.38E+00

7.23E+00

kxf

Dehydration

1.23E-01

4.42E-01

6.51E-01

2.80E-01

kxt

Decomposition

1.70E-01

8.45E-01

3.42E+00

2.32E+00

kxyx

Isomerization

1.02E-02

2.71E-01

4.99E-03

0.00E+00

kxygc

Retro-aldol

2.51E-02

3.85E-02

1.72E-01

2.35E-01

kxyf

Dehydration

0.00E+00

0.00E+00

3.76E-01

3.07E-02

kxyt

Decomposition

0.00E+00

0.00E+00

7.75E-03

9.78E-05

kglgc

Retro-aldol

1.16E+00

2.49E+00

4.44E+00

5.27E+00

kgld

Isomerization

2.98E-01

1.02E+00

2.86E+00

2.68E-01

kglt

Decomposition

1.60E+01

8.63E-01

0.00E+00

8.98E-05

kgct

Decomposition

0.00E+00

1.10E-01

2.09E-01

4.84E-01

kft

Decomposition

0.00E+00

8.50E-06

9.85E-01

9.69E-01

kdgl

Isomerization

0.00E+00

3.68E-01

1.95E+00

0.00E+00

kdt

Decomposition

0.00E+00

0.00E+00

0.00E+00

0.00E+00

kfog

Decomposition

0.00E+00

0.00E+00

0.00E+00

0.00E+00

ktg

Gasification

1.68E-02

3.49E-02

4.77E-02

6.84E-02

6.46E-01

3.15E+00

1.07E+01

1.01E+01

kx

overall xylose decomposition

370


Page 21 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


371

372

Table 4. Activated energy and pre-exponential factor of Arrhenius reactions Reaction

Activated energy (kJ.mol-1)

Pre-exponential factor (s-1)

xgl

86.64

2.069E+07

xygc

56.40

3.001E+03

glgc

35.83

2.328E+03

xt

65.53

2.162E+05

gct

55.25

4.491E+03

tg

31.52

1.347E+01

x

66.67

9.96E+05

373

374



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

375

Table 5. Classification of reactions in SCWG of xylose Radical reaction

Ionic reaction

Arrhenius

Non-Arrhenius

xgl

Retro-aldol condensation

xxy

Isomerization

xygc


xyx

Isomerization

glgc


gld

Isomerization

xt

Decomposition

dgl

Isomerization

gct

Decomposition

xf

Dehydration

tg

gasification

xyf

Dehydration

(ft)

(Decomposition)

glt

Decomposition


Page 22 of 38

Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 6. Sensitivity coefficients of xylose decomposition at temperature of 350 oC and 450 oC

376

xylose

Temperature

xylulose

glyceraldehyde

glycolaldehyde

furfural

dihydroxyacetone

formaldehyde

TOC

gas

[oC]

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

kxxy

350

-0.118

0.879

0.899

1.025

-0.138

0.302

-0.085

-0.007

-0.091

-0.043

-0.055

0.148

-0.085

-0.035

-0.089

-0.041

-0.072

-0.065

kxgl

350

-1.511

-1.069

-0.461

-0.585

0.045

0.185

0.548

0.432

-0.429

-0.497

0.527

0.265

0.586

0.471

-0.186

-0.060

-0.143

-0.124

kxf

350

-0.463

-0.327

-0.139

-0.177

-0.274

-0.204

-0.128

-0.157

0.864

0.842

-0.126

-0.170

-0.118

-0.148

-0.126

-0.149

-0.095

-0.136

kxt

350

-0.875

-0.619

-0.265

-0.337

-0.521

-0.387

-0.244

-0.298

-0.246

-0.286

-0.239

-0.323

-0.225

-0.282

0.515

0.262

0.634

0.395

kxyx

350

0.223

-0.208

-0.225

-1.227

0.073

-0.063

0.015

0.033

0.016

0.041

0.009

-0.045

0.012

0.035

0.015

0.037

0.007

0.030

kxygc

350

-0.007

-0.190

-0.035

-0.204

0.005

0.053

0.005

0.015

0.000

-0.006

0.035

0.123

0.001

0.008

0.000

0.001

0.000

0.000

kxyf

350

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kxyt

350

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kglgc

350

0.000

0.000

0.000

0.000

-1.029

-1.107

0.164

0.115

0.000

0.000

-0.525

-0.701

0.460

0.310

-0.069

-0.055

-0.047

-0.062

kgld

350

0.000

0.000

0.000

0.000

-0.193

0.461

-0.064

-0.018

0.000

0.000

0.782

0.928

-0.174

-0.072

-0.031

-0.023

-0.022

-0.027

kglt

350

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kgct

350

0.000

0.000

0.000

0.000

0.000

0.000

-0.093

-0.519

0.000

0.000

0.000

0.000

0.000

0.000

0.072

0.219

0.041

0.151

kft

350

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kdgl

350

0.000

0.000

0.000

0.000

0.198

-0.148

0.014

0.040

0.000

0.000

-0.213

-1.180

0.036

0.088

0.006

0.022

0.003

0.016

kdt

350

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kfog

350

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

ktg

350

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

-0.030

-0.151

0.982

0.912

377 378



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 24 of 38

Table 6. Sensitivity coefficients of xylose decomposition at temperature of 350 oC and 450 oC (cont.)

379

xylose

Temperature

xylulose

glyceraldehyde

glycolaldehyde

furfural

dihydroxyacetone

formaldehyde

TOC

gas

[oC]

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

tmin

tmax

kxxy

450

-0.174

-1.209

0.975

0.975

-0.045

-0.051

-0.021

0.008

-0.006

0.518

0.062

0.278

-0.022

-0.023

-0.021

-0.015

-0.019

-0.018

kxgl

450

-4.767

-17.127

-0.708

-0.712

-0.389

-0.557

0.255

0.206

-0.756

-0.735

0.226

-0.017

0.301

0.272

-0.318

-0.037

-0.368

-0.117

kxf

450

-0.209

-1.445

-0.028

-0.029

-0.054

-0.062

-0.028

-0.029

0.949

0.426

-0.026

-0.028

-0.027

-0.028

0.008

0.007

0.001

0.010

kxt

450

-1.670

-9.257

-0.233

-0.234

-0.442

-0.500

-0.232

-0.239

-0.249

-0.242

-0.216

-0.227

-0.218

-0.228

0.398

0.043

0.575

0.151

kxyx

450

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kxygc

450

0.000

0.000

-0.144

-1.140

0.000

0.000

0.002

0.009

-0.002

-0.465

0.078

0.158

0.000

0.000

0.000

0.003

0.000

0.002

kxyf

450

0.000

0.000

-0.019

-0.153

0.000

0.000

0.000

-0.003

0.019

0.478

-0.001

-0.018

0.000

0.000

0.000

0.002

0.000

0.001

kxyt

450

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kglgc

450

0.000

0.000

0.000

0.000

-2.640

-14.670

0.052

-0.020

0.000

0.000

-0.739

-0.635

0.153

0.046

0.050

0.016

0.035

0.025

kgld

450

0.000

0.000

0.000

0.000

-0.144

-1.303

-0.022

-0.026

0.000

0.000

0.875

0.663

-0.043

-0.048

-0.005

-0.016

-0.002

-0.013

kglt

450

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kgct

450

0.000

0.000

0.000

0.000

0.000

0.000

-0.255

-2.195

0.000

0.000

0.000

0.000

0.000

0.000

0.298

0.123

0.188

0.257

kft

450

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

-0.576

-2.601

0.000

0.000

0.000

0.000

0.025

0.000

0.019

0.009

kdgl

450

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kdt

450

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

kfog

450

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

ktg

450

0.000

-1.209

0.000

0.975

0.000

-0.051

0.000

0.008

0.000

0.518

0.000

0.278

0.000

-0.023

-0.034

-0.015

0.979

-0.018

380

tmin is 1.23 s at 350 oC and 0.72 s at 450 oC, tmax is 5.71 s at 350 oC and 5.09 s at 450 oC


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


381

382 383

(a) 300 oC

384 385

(b) 350 oC

386



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

387 388

(c) 400 oC

389 390

(d) 450 oC

391

Figure 1. Carbon balance: carbon content in liquid products and gaseous products, for the

392

experiments conducted at the temperature of (a) 300, (b) 350, (c) 400 and (d) 450°C


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393 394

Figure 2. Proposed reaction network of xylose decomposition in sub- and supercritical water

395



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

396

397

398

Figure 3. Retro-aldol condensation of xylose and xylulose

399 400 401


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402 403

404

405 406

(a) 300 oC

407 408

(b) 350 oC



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

409 410

(c) 400 oC

411 412

413 414

(d) 450 oC o

o

o

o

Figure 4. Product yield at temperature of (a) 300 C, (b) 350 C, (c) 400 C, (d) 450 C and pressure of 25 MPa as a function of residence time

415


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416

417 418

(a)

419 420

(b)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

421 422

(c)

423 424

(d)


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425 426

(e)

427 428

(f)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

429 430

(g)

431 432

(h)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


433 434

(i)

435 436

(j)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

437 438

(k)

439 440

(l)


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441 442

(m)

443 444

(n)

445

Figure 5. Arrhenius plot of (a) kxgl (b) kxygc (c) kglgc (d) kxt (e) kgct (f) ktg (g) kx (h) kxxy (i) kxyx

446

(j) kgld (k) kdgl (l) kxf (m) kxyf, and (n) kglt

447



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

448

449 450

Figure 6. Arrhenius plot of rate constant of the total xylose decomposition for literature

451

comparison 13, 15, 16


Page 38 of 38

Decomposition of Xylose in Sub-and Supercritical Water

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